Emission gas cleaning device of internal combustion engine

ABSTRACT

In a system having an oxygen sensor arranged downstream of a NOx storage-reduction catalyst, a constant current is made to flow between sensor electrodes by a constant current circuit provided in the outside of the oxygen sensor, which makes it possible to change an output characteristic of the oxygen sensor. Further, during a lean combustion control of an engine, a sensing responsiveness to a lean component of the oxygen sensor is improved. In this way, when NOx (lean component) is emitted to the downstream of the catalyst, the NOx can be quickly sensed by the oxygen sensor. Meanwhile, during a rich combustion control of the engine, the sensing responsiveness to a rich component of the oxygen sensor is improved. In this way, when HC and CO (rich components) are emitted to the downstream of the catalyst, the HC and the CO can be quickly sensed by the oxygen sensor.

CROSS REFERENCE TO RELATED APPLICATION

This application is based on Japanese Patent Applications No. 2012-22261filed on Feb. 3, 2012, and No. 2012-221944 filed on Oct. 4, 2012, thedisclosures of which are incorporated herein by reference.

TECHNICAL FIELD

The present disclosure relates to an emission gas cleaning device of aninternal combustion engine which has a catalyst for cleaning an emissiongas of the internal combustion engine and which has an emission gassensor arranged downstream of the catalyst or in the catalyst.

BACKGROUND ART

In an emission gas cleaning system of an internal combustion engine, forexample, as described in Patent Literature 1 (Japanese Patent No.399759), there is proposed the following emission gas cleaning system:that is, an exhaust pipe has a catalyst (for example, a three-waycatalyst or a NOx storage-reduction catalyst) arranged therein and hasan emission gas sensor (an air-fuel ratio sensor or an oxygen sensor)arranged upstream or downstream of the catalyst, the catalyst cleaningan emission gas, and the emission gas sensor. The emission gas sensorsenses an air-fuel ratio of an emission gas or senses whether theair-fuel ratio of the emission gas is rich or lean. The air-fuel ratiois fed back on the basis of an output of the emission gas sensor tothereby increase an emission gas cleaning rate of the catalyst.

In the emission gas sensor such as the oxygen sensor, when the air-fuelratio of the emission gas is changed from a rich value to a lean valueor vice versa, the output of the emission gas sensor causes a delay in achange in a sensor output to a change in an actual air-fuel ratio.Hence, it is necessary for improving a sensing responsiveness.

For embodiment, as described in Patent Literature 2 (JP-H8-20414 B),there is proposed the following emission gas cleaning device: that is, agas sensor such as an oxygen sensor has at least one auxiliaryelectrochemical cell built therein and the auxiliary electrochemicalcell is connected to one electrode of the gas sensor; and an impressedcurrent is applied to the auxiliary electrochemical cell to therebyperform an ion pumping, whereby an output characteristic of the gassensor can be changed according to the impressed current and hence thesensing responsiveness of the gas sensor can be improved.

Further, as described in Patent Literature 3 (JP-S56-89051 A), there isproposed also the following emission gas cleaning device: that is, in anoxygen sensor having a sensor element constructed of a barrier filmlayer, a reference-electrode electronic-conductive layer, a solidelectrolyte layer, and a measurement-electrode electronic-conductivelayer, which are laminated on each other, oxygen ions are moved from ameasurement electrode to a reference electrode by current supplied froma DC power source to thereby make an oxygen partial pressure on themeasurement electrode lower than an oxygen partial pressure in anemission gas (gas to be sensed), which makes it possible to sense anair-fuel ratio leaner than a stoichiometric air-fuel ratio [see a column(A) in FIG. 11].

PRIOR ART LITERATURES

-   [Patent Literature 1] Japanese Patent No. 3997599-   [Patent Literature 2] JP-H8-20414 B-   [Patent Literature 3] JP-S56-89051 A

An air-fuel ratio of an emission gas flowing into a catalyst is changedby an operating state or the like of an internal combustion engine, andalso an air-fuel ratio of the emission gas downstream of the catalystand in the catalyst is changed accordingly. However, the emission gascleaning system of the Patent Literature 1 described above does not havea function of changing an output characteristic of the emission gassensor and hence suffers the effect of delay in a change in a sensoroutput to a change in the air-fuel ratio of the emission gas downstreamof the catalyst and in the catalyst and cannot effectively utilize thecatalyst in some cases, so that the emission gas cleaning system cannoteffectively reduce an exhaust emission.

In the Patent Literature 2 described above is disclosed a technique forchanging the output characteristic of the gas sensor. In this technique,however, the gas sensor needs to have the auxiliary electrochemical cellbuilt therein and hence needs to have a sensor structure greatly changedas compared with an ordinary gas sensor not having the auxiliaryelectrochemical cell built therein. Hence, when the technique describedin the Patent Literature 2 is put into practical use, the design of thegas sensor needs to be changed. As a result, the manufacturing cost ofthe gas sensor is increased.

In the technique of Patent Literature 3, an output E of the oxygensensor can be expressed by the following fundamental equation (NernstEquation).

E=(R×T)/(4×F)×ln(P1/P2)

Here, “R” is a gas constant; “T” is an absolute temperature; “F” isFarady constant; “P1” is an oxygen partial pressure on an atmosphere(reference electrode); and “P2” is an oxygen partial pressure on anexhaust (measurement electrode).

Hence, in order to decrease variations in the output “E” of the oxygensensor to thereby stabilize the output “E”, it is important to stabilizean oxygen concentration on the reference electrode to thereby stabilizethe oxygen partial pressure “P1” on the reference electrode.

However, the oxygen sensor of Patent Literature 3 described above hasthe following construction: the reference electrode is not exposed tothe atmosphere, so that oxygen is supplied to the reference electrodefrom the measurement electrode. Hence, there is a possibility that theoxygen sensor suffers the effect of an oxygen concentration on themeasurement electrode and hence cannot keep the oxygen concentration onthe reference electrode constant. For embodiment, in the case where theoxygen sensor is arranged downstream of the catalyst, the oxygenconcentration of the emission gas sensed by the oxygen sensor isextremely decreased in some case. In this case, the oxygen concentrationon the measurement electrode is extremely decreased and hence the oxygencan hardly be supplied to the reference electrode from the measurementelectrode, so that the oxygen concentration on the reference electrodemight not be kept constant [see a column (B) in FIG. 11]. In this way,an output on a rich of the oxygen sensor might become unstable and hencethe sensing accuracy of the oxygen sensor might be decreased.

In the oxygen sensor of the Patent Literature 3 described above, thecurrent is made to flow in such a way as to supply oxygen to thereference electrode from the measurement electrode, whereby an outputcharacteristic curve of the oxygen sensor can be shifted to a lean.However, the reference electrode is not exposed to the atmosphere andhence the oxygen can hardly be supplied to the measurement electrodefrom the reference electrode, so that an output characteristic curve ofthe oxygen sensor can hardly be shifted to a rich [see a column (C) inFIG. 11].

SUMMARY OF INVENTION

Hence, an object of the present disclosure is to solve the problemdescribed above.

According to one aspect of the present disclosure, an emission gascleaning device is applied to an internal combustion engine having acatalyst which cleans an emission gas of an internal combustion engine,and an emission gas sensor which is arranged on a downstream of thecatalyst or in the catalyst and senses a concentration of a specifiedcomponent in the emission gas by a sensor element. The sensor elementhas a solid electrolyte material provided between a pair of sensorelectrodes. One of the sensor electrodes is exposed to the atmosphere.The emission gas cleaning device of the internal combustion engine has aconstant current supply portion for making a constant current flowbetween the sensor electrodes to thereby change an output characteristicof the emission gas sensor. Further, the emission gas cleaning device ofthe internal combustion engine has a current control portion fordetermining a direction of the constant current made to flow between thesensor electrodes according to a change request for changing the outputcharacteristic of the emission gas sensor or according to an operatingstate of the internal combustion engine. The current control portioncontrols the constant current supply portion in such a way that theconstant current flows in the direction determined.

In this configuration, the output characteristic of the emission gassensor can be changed by making the constant current flow between thesensor electrodes by the constant current supply portion. In this case,the emission gas sensor does not need to have the auxiliaryelectrochemical cell or the like built therein, so that the outputcharacteristic of the emission gas sensor can be changed without causinga significant design change and a large increase in cost of the emissiongas sensor.

Further, the direction of the constant current made to flow between thesensor electrodes is determined according to the change request forchanging the output characteristic of the emission gas sensor oraccording to the operating state of the internal combustion engine andthe constant current supply portion is controlled in such a way that theconstant current flows in the direction determined. Hence, even if astate of the emission gas flowing into the catalyst is changed by theoperating state or the like of the internal combustion engine and hencethe state of the emission gas downstream of the catalyst or in thecatalyst is changed, the output characteristic of the emission gassensor can be changed accordingly and hence the sensing responsivenessof the emission gas sensor can be improved. In this way, the catalystcan be effectively utilized without being much affected by a delay in achange in the sensor output to a change in the state of the emission gasdownstream of the catalyst or in the catalyst and hence the exhaustemission can be effectively reduced.

Still further, one sensor electrode (atmosphere sensor electrode) isexposed to the atmosphere, so an oxygen concentration on the atmospheresensor electrode can be kept at a constant value (corresponding to theatmosphere) regardless of the oxygen concentration on the other sensorelectrode (exhaust sensor electrode). Hence, even in the case where theemission gas sensor is arranged downstream of the catalyst, in otherwords, even in the case where the oxygen concentration of the emissiongas sensed by the emission gas sensor is extremely decreased in somecase, variations in the output of the emission gas sensor can bedecreased and hence the output of the emission gas sensor can bestabilized.

Still further, by making the current flow in such a way that oxygen issupplied to the atmosphere sensor electrode from the exhaust sensorelectrode, the output characteristic curve of the emission gas sensorcan be shifted to a lean, whereas by making the current flow in such away that the oxygen is supplied to the exhaust sensor electrode from theatmosphere sensor electrode, the output characteristic curve of theemission gas sensor can be shifted to a rich. Hence, there is presentedalso an advantage that the output characteristic curve of the emissiongas sensor can be shifted to both of the lean and the rich.

According to a second aspect of the present disclosure, an emission gascleaning device is applied to a system having a NOx storage-reductioncatalyst as the catalyst described above, the NOx storage-reductioncatalyst adsorbing NOx in the emission gas when an air-fuel ratio of theemission gas flowing into the catalyst is lean and reducing, cleaning,discharging the NOx adsorbed by the catalyst when the air-fuel ratio ofthe emission gas flowing into the catalyst becomes rich. In this way,the NOx storage-reduction catalyst can be effectively utilized and hencethe exhaust emission can be reduced.

In a system in which an emission gas sensor is arranged downstream ofthe NOx storage-reduction catalyst or in the NOx storage-reductioncatalyst, during a lean combustion control in which an air-fuel ratio ofan air-fuel mixture to be supplied to an internal combustion engine iscontrolled to a lean value, it is recommended to control a constantcurrent supply portion in such a way that a constant current flows in adirection in which a sensing responsiveness to a lean component of theemission gas sensor is improved, whereas during a rich combustioncontrol in which the air-fuel ratio of the air-fuel mixture to besupplied to the internal combustion engine is controlled to a richvalue, it is recommended to control the constant current supply portionin such a way that the constant current flows in a direction in which asensing responsiveness to a rich component of the emission gas sensor isimproved.

During the lean combustion control, the air-fuel ratio of the emissiongas flowing into the NOx storage-reduction catalyst becomes lean andNO_(x) (lean component) in the emission gas is adsorbed by the NOxstorage-reduction catalyst, but when the amount of the NO_(x) adsorbedby the NOx storage-reduction catalyst is increased, there is broughtabout a state in which the NO_(x) in the emission gas is passed throughthe NOx storage-reduction catalyst and is emitted to the downstream ofthe NOx storage-reduction catalyst. For this reason, if the leanresponsiveness of the emission gas sensor (sensing responsiveness of theemission gas sensor to the lean component) is improved during the leancombustion control, when a state in which the amount of the NO_(x)adsorbed by the NOx storage-reduction catalyst is increased and in whichthe NO_(x) (lean component) is emitted to the downstream of the NOxstorage-reduction catalyst is brought about during the lean combustioncontrol, the state can be quickly sensed by the emission gas sensor. Inthis way, when the state in which the NO_(x) is emitted to thedownstream of the NOx storage-reduction catalyst is brought about afterthe lean combustion control is started, the lean combustion control canbe quickly stopped and hence the amount of emission of the NO_(x) can bereduced.

During the rich combustion control, the air-fuel ratio of the emissiongas flowing into the NOx storage-reduction catalyst becomes rich and theNO_(x) adsorbed by the NOx storage-reduction catalyst is reduced,cleaned, and discharged by HC and CO (rich components) in the emissiongas, but when the amount of the NO_(x) adsorbed by the NOxstorage-reduction catalyst is decreased, there is brought about a statein which the HC and the CO in the emission gas is passed through the NOxstorage-reduction catalyst and is emitted to the downstream of the NOxstorage-reduction catalyst. For this reason, if the rich responsivenessof the emission gas sensor (sensing responsiveness of the emission gassensor to the rich component) is improved during the rich combustioncontrol, when a state in which the amount of the NO_(x) adsorbed by theNOx storage-reduction catalyst is decreased and in which the HC and theCO (rich components) are emitted to the downstream of the NOxstorage-reduction catalyst is brought about during the rich combustioncontrol, the state can be quickly sensed by the emission gas sensor. Inthis way, when a state in which the HC and the CO are emitted to thedownstream of the NOx storage-reduction catalyst is brought about afterthe rich combustion control is started, the rich combustion control canbe quickly stopped and hence the amount of emission of the HC and the COcan be reduced.

The present disclosure may be applied to a system having a three-waycatalyst for cleaning CO, HC, and NO_(x) in an emission gas as thecatalyst described above. In this way, the three-way catalyst can beeffectively utilized and the exhaust emission can be reduced.

BRIEF DESCRIPTION OF DRAWINGS

The above-mentioned object, other objects, features, and advantages ofthe present disclosure will be made clearer by the following detaileddescription with reference to the accompanying drawings.

FIG. 1 is a diagram to show a general configuration of an engine controlsystem in an embodiment 1 of the present disclosure.

FIG. 2 is a section view to show a sectional construction of a sensorelement.

FIG. 3 is an electromotive characteristic graph to show a relationshipbetween an air-fuel ratio (excess air ratio λ) of an emission gas and anelectromotive force of a sensor element.

FIG. 4A is a schematic diagram to show a state of a gas component arounda sensor element.

FIG. 4B is a schematic diagram to show a state of a gas component arounda sensor element.

FIG. 5 is a time chart to illustrate a behavior of a sensor output.

FIG. 6A is a schematic diagram to show a state of a gas component arounda sensor element.

FIG. 6B is a schematic diagram to show a state of a gas component arounda sensor element.

FIG. 7 is an output characteristic graph of an oxygen sensor in the casewhere a lean responsiveness and a rich responsiveness are improved.

FIG. 8 is a time chart to show an embodiment of performing a catalystutilizing control.

FIG. 9 is a flow chart to show a processing flow of a catalyst utilizingcontrol routine.

FIG. 10 is a drawing to illustrate an effect of the embodiment 1.

FIG. 11 is a drawing to illustrate a conventional technique.

FIG. 12 is a diagram to show a general configuration of an enginecontrol system of an embodiment 2 of the present invention.

FIG. 13 is a flow chart to show a processing flow of a sensorresponsiveness control routine.

EMBODIMENTS FOR CARRYING OUT INVENTION

Hereinafter, embodiments in which a mode for carrying out the presentdisclosure is embodied will be described.

Embodiment 1

An embodiment 1 of the present disclosure will be described on the basisof FIG. 1 to FIG. 10.

First, a general configuration of an entire engine control system willbe described on the basis of FIG. 1.

An intake pipe 12 of an engine 11 is provided with a throttle valve 13,the opening of which is controlled by a motor or the like, and athrottle opening sensor 14, which senses an opening of the throttlevalve 13 (throttle position). Further, each of cylinders of the engine11 is provided with a fuel injection valve 15 for performing a directinjection or an intake port injection, whereas a cylinder head of theengine 11 has an ignition plug 16 fixed on each of the cylinders. Anair-fuel mixture in each of the cylinders is ignited by a sparkdischarge of each ignition plug 16.

On the other hand, an exhaust pipe 17 of the engine 11 has a three-waycatalyst 18, which cleans CO, HC, and NO_(x) in an emission gas,provided therein and has a NOx storage-reduction catalyst 19 provideddownstream of the three-way catalyst 18. The NOx storage-reductioncatalyst 19 has the following characteristic: that is, when an air-fuelratio of the emission gas flowing into the NOx storage-reductioncatalyst 19 is lean, the NOx storage-reduction catalyst 19 adsorbsNO_(x) in the emission gas, whereas when the air-fuel ratio of theemission gas flowing into the NOx storage-reduction catalyst 19 becomesrich, the NOx storage-reduction catalyst 19 reduces, cleans, anddischarges NO_(x) adsorbed by the NOx storage-reduction catalyst 19.

Further, the exhaust pipe 17 has emission gas sensors 20, 21, each ofwhich senses the air-fuel ratio of the emission gas or senses whetherthe air-fuel ratio of the emission gas is rich or lean, providedupstream and downstream of the catalyst 18. As each of the emission gassensors 20, 21 is used an air-fuel ratio sensor (linear NF sensor) foroutputting a linear air-fuel ratio signal corresponding to the air-fuelratio of the emission gas or an oxygen sensor (O₂ sensor), the outputvoltage of which is reversed depending on whether the air-fuel ratio ofthe emission gas is rich or lean with respect to a stoichiometricair-fuel ratio. Further, the exhaust pipe 17 has an oxygen sensor (O₂sensor) 28, arranged as an emission gas sensor downstream of the NOxstorage-reduction catalyst 19. The output voltage of the oxygen sensoris reversed depending on whether the air-fuel ratio of the emission gasis rich or lean with respect to the stoichiometric air-fuel ratio.

Still further, the present system has various sensors such as a crankangle sensor 22 for outputting a pulse signal every time a crankshaft(not shown in the drawing) of the engine 11 is rotated by a specifiedcrank angle, an air-flow sensor 23 for sensing an intake air volume ofthe engine 11, and a coolant temperature sensor 24 for sensing a coolanttemperature of the engine 11. A crank angle and an engine rotation speedare sensed on the basis of an output signal of the crank angle sensor22.

The outputs of these various sensors are inputted to an electroniccontrol unit (hereinafter denoted by “ECU”) 25. The ECU 25 is mainlyconstructed of a microcomputer and executes various programs, which arestored in a built-in ROM (memory medium) and are used for controllingthe engine, to thereby control a fuel injection quantity, an ignitiontiming, a throttle opening (intake air volume), and the like accordingto an engine operating state.

Next, the construction of the oxygen sensor 28 will be described on thebasis of FIG. 2.

The oxygen sensor 28 has a sensor element 31 of a cup type structure. Inreality, the sensor element 31 is constructed in such a way that thewhole of the element is housed in a housing or an element cover notshown in the drawing and is arranged in the exhaust pipe 17 of theengine 11.

In the sensor element 31, a solid electrolyte layer 32 (solidelectrolyte material) is formed in the shape of a cup when viewed in across section and has an exhaust electrode layer 33 fixed on its outersurface and has an atmosphere electrode layer 34 fixed on its innersurface. The solid electrolyte layer 32 is formed of an oxygen ionconductive oxide sintered material in which CaO, MgO, Y₂O₃, or Yb₂O₃ isdissolved as a stabilizer in ZrO₂, HfO₂, ThO₂, or Bi₂O₃. Further, eachof the electrode layers 33, 34 is formed of a noble metal such asplatinum having an enhanced catalytic activity and has porous chemicalplating or the like applied to its surface. These electrode layers 33,34 form a pair of opposite electrodes (sensor electrodes). An innerspace surrounded by the solid electrolyte layer 32 becomes an atmospherechamber 35 and the atmosphere chamber 35 has a heater 36 housed therein.The heater 36 has a heating capacity sufficient for activating thesensor element 31 and the whole of the sensor element 31 is heated bythe heating energy of the heater 36. An activation temperature of theoxygen sensor 28 is, for example, approximately 350 to 400° C. Here, theatmosphere chamber 35 has the atmosphere introduced thereinto and hencehas its interior held at a specified oxygen concentration, whereby theatmosphere electrode layer 34 is exposed to the atmosphere in theatmosphere chamber 35.

In the sensor element 31, the outside (electrode layer 33) of the solidelectrolyte layer 32 is in an exhaust atmosphere and the inside(electrode layer 34) of the solid electrolyte layer 32 is in theatmosphere, whereby an electromotive force is generated between theelectrode layers 33, 34 according to a difference in the oxygenconcentration (a difference in an oxygen partial pressure) between theseatmospheres. In other words, in the sensor element 31, a differentelectromotive force is generated according to whether the air-fuel ratiois rich or lean. In this way, the oxygen sensor 28 outputs anelectromotive force signal corresponding to the oxygen concentration(that is, the air-fuel ratio) of the emission gas.

As shown in FIG. 3, the sensor element 31 generates a differentelectromotive force according to whether the air-fuel ratio is rich orlean with respect to a stoichiometric air-fuel ratio (excess air ratioλ=1) and has a characteristic such that the electromotive force issuddenly changed near the stoichiometric air-fuel ratio (excess airratio λ=1). Specifically, when the air-fuel ratio is rich, theelectromotive force generated by the sensor element 31 is approximately0.9 V, whereas when the air-fuel ratio is lean, the electromotive forcegenerated by the sensor element 31 is approximately 0 V.

As shown in FIG. 2, the sensor element 31 has the exhaust electrodelayer 33 grounded to the earth and has the atmosphere electrode layer 34connected to a microcomputer 26. When the sensor element 31 generates anelectromotive force according to the air-fuel ratio (the oxygenconcentration) of the emission gas, a sensor sensing signalcorresponding to the electromotive force is outputted to themicrocomputer 26. The microcomputer 26 is built in, for example, the ECU25 and calculates the air-fuel ratio on the basis of the sensor sensingsignal. Here, the microcomputer 26 may calculate an engine rotationspeed or an intake air volume on the basis of the sensed results of thevarious sensors described above.

By the way, when the engine 11 is operated, an actual air-fuel ratio ofthe emission gas is successively varied and, in some cases, isrepeatedly varied between a rich value and a lean value. When the actualair-fuel ratio is varied in this way, if the sensing responsiveness ofthe oxygen sensor 28 is low, it is concerned that the low sensingresponsiveness will cause a bad effect on the performance of the engine11. For embodiment, it is concerned that when the engine 11 is operatedat a high load, the amount of NO_(x) in the emission gas will beincreased more than expected.

The sensing responsiveness of the oxygen sensor 28 when the actualair-fuel ratio is varied between the rich value and the lean value willbe described. When the actual air-fuel ratio (actual air-fuel ratiodownstream of the NOx storage-reduction catalyst 19) is varied betweenthe rich value and the lean value in the emission gas emitted from theengine 11, the component composition of the emission gas is changed. Atthis time, when the component of the emission gas just before thecomponent composition of the emission gas being changed remains, achange in the output of the oxygen sensor 28 to the air-fuel ratio afterthe component composition of the emission gas being changed (that is,the responsiveness of the sensor output) becomes slow. Specifically,when the actual air-fuel ratio is changed from the rich value to thelean value, as shown in FIG. 4A, just after the actual air-fuel ratio ischanged to the lean value, HC or the like that is a rich componentremains near the exhaust electrode layer 33 and hence the reaction of alean component (NO_(x) or the like) at the sensor electrode is preventedby the rich component. As a result, the oxygen sensor 28 is lowered inthe responsiveness of a lean output. On the other hand, when the actualair-fuel ratio is changed from the lean value to the rich value, asshown in FIG. 4B, just after the actual air-fuel ratio is changed to therich value, NOx or the like which is a lean component remains near theexhaust electrode layer 33 and hence the reaction of a rich component(HC or the like) at the sensor electrode is prevented by the leancomponent. As a result, the oxygen sensor 28 is lowered in theresponsiveness of a rich output.

A change in the output of the oxygen sensor 21 will be described by theuse of a time chart shown in FIG. 5. In FIG. 5, when the actual air-fuelratio is changed between a rich value and a lean value, a sensor output(output of the oxygen sensor 28) is changed between a rich gas sensingvalue (0.9 V) and a lean gas sensing value (0 V) according to a changein the actual air-fuel ratio. However, in this case, the sensor outputis changed with a delay to the change in the actual air-fuel ratio. InFIG. 5, when the actual air-fuel ratio is changed from the rich value tothe lean value, the sensor output is changed with a delay of TD1 to thechange in the actual air-fuel ratio, whereas when the actual air-fuelratio is changed from the lean value to the rich value, the sensoroutput is changed with a delay of TD2 to the change in the actualair-fuel ratio.

Hence, in the present embodiment, as shown in FIG. 2, a constant currentcircuit 27 as a constant current supply portion is connected to theatmosphere electrode layer 34 and the microcomputer 26 controls thesupply of a constant current “Ics” by the constant current circuit 27 tothereby make the current flow in a specified direction between the pairof sensor electrodes (exhaust electrode layer 33 and the atmosphereelectrode layer 34), which in turn changes the output characteristic ofthe oxygen sensor 28 to thereby change the sensing responsiveness of theoxygen sensor 28. In this case, the microcomputer 26 sets a directionand a quantity of the constant current “Ics” flowing between the pair ofsensor electrodes and controls the constant current circuit 27 in such away that the constant current “Ics” having the direction and thequantity set flows.

In more detail, the constant current circuit 27 is a circuit thatsupplies the atmosphere electrode layer 34 with the constant current“Ics” in either of a forward direction or a reverse direction and thatcan variably adjust the flow rate of the constant current “Ics”. Inother words, the microcomputer 26 variably controls the constant current“Ics” by a PWM control. In this case, in the constant current circuit27, the constant current “Ics” is adjusted according to a duty signaloutputted from the microcomputer 26 and the constant current “Ics”having its flow rate controlled is made to flow between the sensorelectrodes (between the exhaust electrode layer 33 and the atmosphereelectrode layer 34).

In the present embodiment, the constant current “Ics” flowing in thedirection from the exhaust electrode layer 33 to the atmosphereelectrode layer 34 is assumed to be a negative constant current(−“Ics”), whereas the constant current “Ics” flowing in the directionfrom the atmosphere electrode layer 34 to the exhaust electrode layer 33is assumed to be a positive constant current (+“Ics”).

For embodiment, in the case where the sensing responsiveness (leansensitivity) when the actual air-fuel ratio is changed from the richvalue to the lean value is improved, as shown in FIG. 6A, the constantcurrent “Ics” (negative constant current “Ics”) is made to flow in sucha way that oxygen is supplied from the atmosphere electrode layer 34 tothe exhaust electrode layer 33 through the solid electrolyte layer 32.In this case, the oxygen is supplied to the exhaust from the atmosphere,whereby an oxidation reaction of the rich component (HO) existing(remaining) around the exhaust electrode layer 33 is accelerated andhence the rich component can be quickly removed by the acceleratedoxidation reaction. In this way, the lean component (NO_(x)) can beeasily reacted in the exhaust electrode layer 33, which results inimproving the responsiveness of the lean output of the oxygen sensor 28.

On the other hand, in the case where the sensing responsiveness (richsensitivity) when the actual air-fuel ratio is changed from the leanvalue to the rich value is improved, as shown in FIG. 6B, the constantcurrent “Ics” (positive constant current “Ics”) is made to flow in sucha way that oxygen is supplied from the exhaust electrode layer 33 to theatmosphere electrode layer 34 through the solid electrolyte layer 32. Inthis case, the oxygen is supplied to the atmosphere from the exhaust,whereby a reduction reaction of the lean component (NO_(x)) existing(remaining) around the exhaust electrode layer 33 is accelerated andhence the lean component can be quickly removed by the acceleratedreduction reaction. In this way, the rich component (HC) can be easilyreacted in the exhaust electrode layer 33, which results in improvingthe responsiveness of the rich output of the oxygen sensor 28.

FIG. 7 is a graph to show an output characteristic (electromotive forcecharacteristic) of the oxygen sensor 28 in the case where the sensingresponsiveness (lean sensitivity) when the actual air-fuel ratio ischanged from the rich value to the lean value and in the case where thesensing responsiveness (rich sensitivity) when the actual air-fuel ratiois changed from the lean value to the rich value.

In order to improve the sensing responsiveness (lean sensitivity) in acase where the actual air-fuel ratio is changed from the rich value tothe lean value as described above, when the negative constant current“Ics” is made to flow in such a way that the oxygen is supplied from theatmosphere electrode layer 34 to the exhaust electrode layer 33 throughthe solid electrolyte layer 32 (see FIG. 6A), an output characteristiccurve is shifted to a rich (in more detail, to the rich and to the inwhich the electromotive force is decreased) as shown by a single dot anddash line (a) in FIG. 7. In this case, even if the actual air-fuel ratiois in a rich region near the stoichiometric air-fuel ratio, the sensoroutput becomes a lean output. The output characteristic of the oxygensensor 28 is improved in the sensing responsiveness (lean sensitivity)when the actual air-fuel ratio is changed from the rich value to thelean value.

On the other hand, in order to improve the sensing responsiveness (richsensitivity) in a case where the actual air-fuel ratio is changed fromthe lean value to the rich value as described above, when the positiveconstant current “Ics” is made to flow in such a way that the oxygen issupplied from the exhaust electrode layer 33 to the atmosphere electrodelayer 34 through the solid electrolyte layer 32 (see FIG. 6B), theoutput characteristic curve is shifted to a lean (in more detail, theoutput characteristic curve is shifted to the lean and to the in whichthe electromotive force is increased) as shown by a single dot and dashline (b) in FIG. 7. In this case, even if the actual air-fuel ratio iswithin a lean region near the stoichiometric air-fuel ratio, the sensoroutput becomes a rich output. That is, the output characteristic of theoxygen sensor 28 is improved in the sensing responsiveness (leansensitivity) when the actual air-fuel ratio is changed from the leanvalue to the rich value.

In the present embodiment 1, the ECU 25 (or the microcomputer 26)performs a catalyst utilizing control routine shown in FIG. 9, whichwill be described later, thereby determining the direction of theconstant current “Ics” flowing between the sensor electrodes (betweenthe exhaust electrode layer 33 and the atmosphere electrode layer 34)according to the operating state of the engine 11 and controlling theconstant current circuit 27 in such a way that the constant current“Ics” flows in the determined direction. In this way, even if a state ofthe emission gas flowing into the NOx storage-reduction catalyst 19 ischanged by the operating state of the engine 11 to thereby cause achange in the state of the emission gas downstream of the NOxstorage-reduction catalyst 19, the ECU 25 (or the microcomputer 26)changes the output characteristic of the oxygen sensor 28 according tothe change in the state of the emission gas and hence can improve thesensing responsiveness of the oxygen sensor 28.

Specifically, as shown by a time chart in FIG. 8, when a specified leanoperation performance condition is met while the engine 11 is beingoperated, a lean combustion control for controlling the air-fuel ratioof an air-fuel mixture to be supplied to the engine 11 to a leaner valuethan the stoichiometric air-fuel ratio (λ=1) to thereby combust theair-fuel mixture at a lean air-fuel ratio is performed. At a timing t1when the output of the oxygen sensor 28 becomes not more than aspecified lean determination threshold value (for example, 0.45 V)during the lean combustion control, it is determined that NO_(x) (leancomponent) starts to be emitted to the downstream of the NOxstorage-reduction catalyst 28. Then, the lean combustion control isstopped and a rich combustion control for controlling the air-fuel ratioof the air-fuel mixture to be supplied to the engine 11 to a richervalue than the stoichiometric air-fuel ratio (λ=1) to thereby combustthe air-fuel mixture at a rich air-fuel ratio is performed. At a timingt2 when the output of the oxygen sensor 28 becomes not less than aspecified rich determination threshold value (for example, 0.45 V)during the rich combustion control, it is determined that HC and CO(rich components) start to be emitted to the downstream of the NOxstorage-reduction catalyst 28. Then, the rich combustion control isstopped and the lean combustion control is performed. In this way, thelean combustion control and the rich combustion control are alternatelyperformed.

At that time, during the lean combustion control, the constant currentcircuit 27 is controlled in such a way that the constant current “Ics”flows in the direction in which the lean sensitivity of the oxygensensor 28 is improved to thereby improve the lean responsiveness(sensing responsiveness to the lean component). In this case, theconstant current circuit 27 is controlled in such a way that theconstant current “Ics” (negative constant current “Ics”) flows in thedirection in which the oxygen is supplied from the atmosphere electrodelayer 34 to the exhaust electrode layer 33. On the other hand, duringthe rich combustion control, the constant current circuit 27 iscontrolled in such a way that the constant current “Ics” flows in thedirection in which the rich sensitivity of the oxygen sensor 28 isimproved to thereby improve the rich responsiveness (sensingresponsiveness to the rich component). In this case, the constantcurrent circuit 27 is controlled in such a way that the constant current“Ics” (positive constant current “Ics”) flows in the direction in whichthe oxygen is supplied from the exhaust electrode layer 33 to theatmosphere electrode layer 34.

During the lean combustion control, the air-fuel ratio of the emissiongas flowing into the NOx storage-reduction catalyst 19 becomes lean andNO_(x) (lean component) in the emission gas is adsorbed by the NOxstorage-reduction catalyst 19. However, when the amount of NO_(x)adsorbed by the NOx storage-reduction catalyst 19 is increased, NO_(x)in the emission gas passes through the NOx storage-reduction catalyst 19and is emitted to the downstream of the NOx storage-reduction catalyst19. For this reason, if the lean responsiveness (sensing responsivenessto the lean component) of the oxygen sensor 28 is improved during thelean combustion control, when the amount of NO_(x) adsorbed by the NOxstorage-reduction catalyst 19 is increased during the lean combustioncontrol to thereby bring about a state in which the NO_(x) (leancomponent) is emitted to the downstream of the NOx storage-reductioncatalyst 19, the state can be quickly sensed by the oxygen sensor 28. Inthis way, when there is brought about the state in which the NO_(x) isemitted to the downstream of the NOx storage-reduction catalyst 19 afterthe lean combustion control is started, the lean combustion control canbe quickly stopped. Hence, the amount of emission of NO_(x) can bereduced as compared with a conventional system not having a function ofchanging a sensor output characteristic (see broken lines in FIG. 8).

On the other hand, during the rich combustion control, the air-fuelratio of the emission gas flowing into the NOx storage-reductioncatalyst 19 becomes rich and the NO_(x) adsorbed by the NOxstorage-reduction catalyst 19 is reduced, cleaned, and discharged by HCand CO (rich components) in the emission gas. However, when the amountof NO_(x) adsorbed by the NOx storage-reduction catalyst 19 isdecreased, the HC and the CO in the emission gas pass through the NOxstorage-reduction catalyst 19 and are emitted to the downstream of theNOx storage-reduction catalyst 19. For this reason, if the richresponsiveness (sensing responsiveness to the rich component) of theoxygen sensor 28 is improved during the rich combustion control, whenthe amount of NO_(x) adsorbed by the NOx storage-reduction catalyst 19is decreased during the rich combustion control to thereby bring about astate in which the HC and the CO (rich components) are emitted to thedownstream of the NOx storage-reduction catalyst 19, the state can bequickly sensed by the oxygen sensor 28. In this way, when there isbrought about the state in which the HC and the CO are emitted to thedownstream of the NOx storage-reduction catalyst 19 after the richcombustion control is started, the rich combustion control can bequickly stopped. Hence, the amount of emission of the HC and the CO canbe reduced as compared with the conventional system not having thefunction of changing the sensor output characteristic (see broken linesin FIG. 8).

Hereinafter, processing contents of the catalyst utilizing controlroutine shown in FIG. 9 performed by the ECU 25 (or the microcomputer26) in the present embodiment will be described.

The catalyst utilizing control routine shown in FIG. 9 is repeatedlyperformed at a specified period during a period in which the power ofthe ECU 25 is on, thereby playing a role as a current control portion.First, in step 101, it is determined whether or not a lean operationperformance condition is met. The lean operation performance conditionis to satisfy, for example, all of the following conditions (1) to (3).

(1) A coolant temperature of the engine 11 is not less than a specifiedtemperature.

(2) The NOx storage-reduction catalyst 19 is in an active state (forexample, an estimated temperature or a sensed temperature of thecatalyst 19 is not less than an active temperature, or time which passesafter the engine is started is not less than a specified time.)

(3) Each portion of the system (for example, fuel system and exhaustsystem) is not abnormal.

If all of these conditions (1) to (3) are satisfied, the lean operationperformance condition is met. However, if any one of the conditions (1)to (3) is not satisfied, the lean operation performance condition is notmet.

In the case where it is determined in this step 101 that the leanoperation performance condition is met, pieces of processing in steps102 to 108 are repeatedly performed. First, the routine proceeds to step102 in which the lean combustion control for controlling the air-fuelratio of the air-fuel mixture to be supplied to the engine 11 to aleaner value than the stoichiometric air-fuel ratio (λ=1) to therebycombust the air-fuel mixture at the lean air-fuel ratio is performed.

Then, the routine proceeds to step 103 in which the constant currentcircuit 27 is controlled during the lean combustion control in such away that the constant current “Ics” flows in the direction in which thelean responsiveness of the oxygen sensor 28 is improved. In other words,the constant current circuit 27 is controlled in such a way that theconstant current “Ics” (negative constant current “Ics”) flows in adirection in which the oxygen is supplied from the atmosphere electrodelayer 34 to the exhaust electrode layer 33. In this way, the leanresponsiveness of the oxygen sensor 28 is improved.

Then, the routine proceeds to step 104 in which it is determined whetheror not the output of the oxygen sensor 28 is not more than the leandetermination threshold value (for example, 0.45 V). In the case whereit is determined that the output of the oxygen sensor 28 is more thanthe lean determination threshold value, the routine returns to step 102.Then, the lean combustion control is continuously performed and acontrol for increasing the lean responsiveness of the oxygen sensor 28is continuously performed (steps 102, 103).

Then, at a timing when it is determined in the step 104 that the outputof the oxygen sensor 28 is not more than the lean determinationthreshold value, it is determined that the NOx (lean component) startsto be emitted to the downstream of the NOx storage-reduction catalyst19. Then, the routine proceeds to step 105 in which the lean combustioncontrol is stopped and the rich combustion control for controlling theair-fuel ratio of the air-fuel mixture to be supplied to the engine 11to a richer value than the stoichiometric air-fuel ratio (λ=1) tothereby combust the air-fuel mixture at the rich air-fuel ratio isperformed.

Then, the routine proceeds to step 106 in which the constant currentcircuit 27 is controlled during the rich combustion control in such away that the constant current “Ics” flows in the direction in which therich responsiveness of the oxygen sensor 28 is improved. In other words,the constant current circuit 27 is controlled in such a way that theconstant current “Ics” (positive constant current “Ics”) flows in thedirection in which the oxygen is supplied to the atmosphere electrodelayer 34 from the exhaust electrode layer 33. In this way, the richresponsiveness of the oxygen sensor 28 is improved.

Then, the routine proceeds to step 107. Then, if it is determined bywhether or not the output of the oxygen sensor 28 is not less than therich determination value (for example, 0.45 V) that the output of theoxygen sensor 28 is less than the rich determination threshold value,the routine returns to the step 105 in which the rich combustion controlis continuously performed and a control for increasing the richresponsiveness of the oxygen sensor 28 is continuously performed (steps105, 106).

Then, at a timing when it is determined in step 107 that the output ofthe oxygen sensor 28 is not less than the rich determination thresholdvalue, it is determined that HC and CO (rich components) start to beemitted to the downstream of the NOx storage-reduction catalyst 19 andthen the routine proceeds to step 108 in which the rich combustioncontrol is stopped.

On the other hand, in the case where it is determined in the step 101that the lean operation performance condition is not met, the routineproceeds to step 109 in which a stoichiometric air-fuel ratio combustioncontrol for controlling the air-fuel ratio of the air-fuel mixture to besupplied to the engine 11 to a stoichiometric air-fuel ratio to therebycombust the air-fuel mixture at the stoichiometric air-fuel ratio isperformed. Then, the routine proceeds to step 110 in which a control fornot changing the sensing responsiveness of the oxygen sensor 28 withrespect to a reference responsiveness, that is, a control forcontrolling the constant current “Ics” to “0” is performed.

In the present embodiment 1 described above, in the system having theoxygen sensor 28 arranged downstream of the NOx storage-reductioncatalyst 19, the constant current is made to flow between the sensorelectrodes by the constant current circuit 27 provided in the outside ofthe oxygen sensor 28, whereby the output characteristic of the oxygensensor 28 can be changed and hence the lean responsiveness and the richresponsiveness of the oxygen sensor 28 can be improved. In addition, theoxygen sensor 28 does not need to have the auxiliary electrochemicalcell or the like built therein, so that the output characteristic of theoxygen sensor 28 can be changed without causing a significant designchange and an increase in cost of the oxygen sensor 28.

Further, during the lean combustion control, the constant currentcircuit 27 is controlled in such a way that the constant current “Ics”flows in the direction in which the lean responsiveness of the oxygensensor 28 is improved. Hence, when there is brought about a state inwhich the NO_(x) (lean component) is emitted to the downstream of theNOx storage-reduction catalyst 19, the state can be quickly sensed bythe oxygen sensor 28 and the lean combustion control can be quicklystopped, whereby the amount of emission of the NO_(x) can be reduced. Onthe other hand, during the rich combustion control, the constant currentcircuit 27 is controlled in such a way that the constant current “Ics”flows in the direction in which the rich responsiveness of the oxygensensor 28 is improved. Hence, when there is brought about a state inwhich the HC and the CO (rich components) are emitted to the downstreamof the NOx storage-reduction catalyst 19, the state can be quicklysensed by the oxygen sensor 28 and the rich combustion control can bequickly stopped, whereby the amount of emission of the HC and the CO canbe reduced. In this way, the NOx storage-reduction catalyst 19 can beeffectively utilized without being much affected by the delay of achange in the output of the oxygen sensor 28 to a change of the state ofthe emission gas downstream of the NOx storage-reduction catalyst 19 andhence the exhaust emission can be effectively reduced.

By the way, the output E of the oxygen sensor 28 can be expressed by thefollowing fundamental equation (Nernst Equation).

E=(R×T)/(4×F)×ln(P1/P2)

Here, “R” is a gas constant; “T” is an absolute temperature; “F” isFarady constant; “P1” is an oxygen partial pressure on the atmosphereelectrode layer 34; and “P2” is an oxygen partial pressure on an exhaustelectrode layer 33.

Hence, in order to decrease variations in the output E of the oxygensensor 28 to thereby stabilize the output E, it is important tostabilize an oxygen concentration on the atmosphere electrode layer 34to thereby stabilize the oxygen partial pressure P1 on the atmosphereelectrode layer 34.

In this regard, the oxygen sensor 28 of the present embodiment 1, asshown in FIG. 10, has the atmosphere electrode layer 34 exposed to theatmosphere, so that the oxygen sensor 28 can always keep the oxygenconcentration on the atmosphere electrode layer 34 at a constant value(corresponding to the atmosphere) regardless of the oxygen concentrationon the exhaust electrode layer 33. Hence, even in the case where theoxygen sensor 28 is arranged downstream of the catalyst 19, in otherwords, even in the case where the oxygen concentration of the emissiongas sensed by the oxygen sensor 28 is significantly decreased, theoutput of the oxygen sensor 28 can be stabilized.

Further, the output characteristic curve of the oxygen sensor 28 can beshifted to the lean by making the current flow in such a way that theoxygen is supplied to the atmosphere electrode layer 34 from the exhaustelectrode layer 33, whereas the output characteristic curve of theoxygen sensor 28 can be shifted to the rich by making the current flowin such a way that the oxygen is supplied to the exhaust electrode layer33 from the atmosphere electrode layer 34. In other words, there ispresented also an advantage that the output characteristic curve of theoxygen sensor 28 can be shifted to either of the lean and the rich.

In this regard, the embodiment 1 employs the configuration in which theoxygen sensor 28 is arranged downstream of the NOx storage-reductioncatalyst 19. However, it is also recommended to employ a configurationin which that the oxygen sensor 28 is arranged at a position in the NOxstorage-reduction catalyst 19 (for example, at a middle position betweenan inlet and an outlet of the catalyst 19).

Embodiment 2

Next, an embodiment 2 of the present disclosure will be described by theuse of FIG. 12 and FIG. 13. However, descriptions of the partssubstantially identical to those in the embodiment 1 will be omitted orsimplified and parts different from those in the embodiment 1 will bemainly described.

In the present embodiment 2, a three-way catalyst 37 for cleaning CO,HC, and NO_(x) in the emission gas is arranged also downstream of thethree-way catalyst 18. Further, an emission gas sensor 20 (air-fuelratio sensor or oxygen sensor) for sensing an air-fuel ratio of anemission gas or sensing whether an air-fuel ratio of an emission gas isrich or lean is arranged upstream of the three-way catalyst 18, and anoxygen sensor 28, the output voltage of which is reversed depending onwhether the air-fuel ratio of the emission gas is rich or lean withrespect to a stoichiometric air-fuel ratio, is arranged downstream ofthe three-way catalyst 18 (between the three-way catalyst 18 and thethree-way catalyst 37).

Further, in the present embodiment 2, an ECU 25 (or microcomputer 26)performs a sensor responsiveness control routine shown in FIG. 13, whichwill be described later.

The sensor responsiveness control routine shown in FIG. 13 is repeatedlyperformed at a specified period during a period in which the power ofthe ECU 25 is on. In the sensor responsiveness control routine, it isdetermined in steps 201 to 203 whether or not a change request forchanging a responsiveness of the oxygen sensor 28 is made, and in steps204 to 207, a constant current control is performed on the basis of adetermination result of the change request to thereby change theresponsiveness of the oxygen sensor 28.

In step 201, it is determined whether an engine 11 is in a cold stateaccording to whether or not any one of the following conditions (1) to(3) is satisfied.

(1) A coolant temperature of the engine 11 is not more than a specifiedtemperature.

(2) An oil temperature (temperature of lubricating oil) of the engine 11is not more than a specified temperature.

(3) A fuel temperature in a fuel passage is not more than a specifiedtemperature.

In the case where it is determined in this step 201 that the engine 11is in the clod state, it is determined that a change request forincreasing a rich responsiveness (sensing responsiveness when theair-fuel ratio of the emission gas is changed to a rich value) is made.In this case, the routine proceeds to step 204 in which the supply of aconstant current “Ics” is controlled on the basis of the change requestfor increasing the rich responsiveness. Specifically, “a positiveconstant current Ics” is set as a constant current of a constant currentcircuit 27. At this time, the constant current circuit 27 is controlledby the microcomputer 26 and the constant current “Ics” (positiveconstant current “Ics”) flows in the direction in which oxygen issupplied to the atmosphere electrode layer 34 from the exhaust electrodelayer 33. In this way, in the case where the engine 11 is in the coldstate, the rich responsiveness of the oxygen sensor 28 is improved.Here, it is recommended that the amount of the constant current be aspecified value determined previously.

On the other hand, in the case where it is determined in step 201 thatthe engine 11 is not in the cold state, the routine proceeds to step 202in which it is determined whether or not the engine 11 is in a high loadoperating state according to whether or not any one of the followingconditions (4) to (6) is satisfied.

(4) An air volume introduced into a cylinder is not less than aspecified volume.

(5) A combustion pressure in a cylinder is not less than a specifiedvalue.

(6) An accelerator opening is not less than a specified value.

In the case where it is determined in this step 202 that the engine 11is in the high load operating state, it is determined that a changerequest for increasing the lean responsiveness (sensing responsivenesswhen the air-fuel ratio of the emission gas is changed to a lean value)is made. In this case, the routine proceeds to step 205 in which thesupply of the constant current “Ics” is controlled on the basis of thechange request for increasing the lean responsiveness. Specifically, “anegative constant current “Ics” is set as the constant current of theconstant current circuit 27. At this time, the constant current circuit27 is controlled by the microcomputer 26, whereby the constant current“Ics” (negative constant current “Ics”) flows in the direction in whichthe oxygen is supplied to the exhaust electrode layer 33 from atmosphereelectrode layer 34. In this way, in the case where the engine 11 is inthe high load operating state, the lean responsiveness of the oxygensensor 28 is improved. Here, it is recommended that the amount of theconstant current be a specified value determined previously.

When a high load operating period in which the engine 11 is operated inthe high load operating state is considered, the high load operatingperiod includes a transient period in which an engine load is changed onan increase and a high-load steady period in which the engine 11 isincreased in load increased and is brought into a high load state. Inthis case, the lean responsiveness is improved in both of the transientperiod and the high-load steady period, and at the time of increasingthe sensing responsiveness, it is recommended to make a responsivenesslevel required as the sensing responsiveness different between in thetransient period and in the high-load steady period.

Specifically, the responsiveness level in the transient period is madehigher than the responsiveness level in the high-load steady period. Inother words, in the case where it is determined that the engine 11 is inthe high-load operating state, that is, in the high-load operatingperiod, it is further determined whether the high-load operating periodis the transient period or the high-load steady period. A determinationthat the high-load operating period is the transient period correspondsto a determination that a change request for increasing the leanresponsiveness and for comparatively decreasing the responsiveness level(decreasing the responsiveness level as compared with the responsivenesslevel when the high-load operating period is the high-load steadyperiod) is made. On the other hand, a determination that the high-loadoperating period is the high-load steady period corresponds to adetermination that a change request for increasing the leanresponsiveness and for comparatively increasing the responsiveness level(increasing the responsiveness level as compared with the responsivenesswhen the high-load operating period is the transient period) is made. Ineach of the case where the high-load operating period is the transientperiod and the case where high-load operating period is the high-loadsteady period, the supply of the constant current “Ics” is controlled onthe basis of the change request.

On the other hand, in the case where it is determined in the step 202that the engine 11 is not in the high-load operating state, the routineproceeds to step 203 in which it is determined whether or not thepresent timing is just after the engine 11 is returned to a fuelinjecting operation from a fuel cutting operation and whether or not arich injection control for neutralizing both catalysts 18, 19 isperformed. This rich injection control is an air-fuel ratio control fortemporally enriching an air-fuel ratio so as to resolve a state in whichboth catalysts 18, 37 are excessive of air (in an extremely leanatmosphere) on the basis of the sensed result of the oxygen sensor 28when the engine 11 is returned from the fuel cutting operation. In therich injection control, the atmospheres of both catalysts 18, 37 areneutralized (brought into a state in which the atmosphere is held nearat the stoichiometric air-fuel ratio) by enriching the amount of fuelinjected. Then, when the output of the oxygen sensor 28 is shifted froma lean value to a rich value after the engine 11 is returned from thefuel cutting operation, the rich injection control is finished. In thepresent embodiment, in the case where the rich injection control isperformed, the sensing responsiveness when the air-fuel ratio of theemission gas is changed to the rich value is decreased.

In the case where it is determined in this step 203 that the richinjection control is performed, it is determined that a change requestfor decreasing the rich responsiveness (sensing responsiveness when theair-fuel ratio of the emission gas is changed to the rich value) ismade. In this case, the routine proceeds to step 206 in which the supplyof the constant current “Ics” is controlled on the basis of the changerequest for deteriorating the rich responsiveness. Specifically, “anegative constant current Ics” is set as the constant current of theconstant current circuit 27. At this time, the constant current circuit27 is controlled by the microcomputer 26, whereby the constant current“Ics” (negative constant current “Ics”) flows in the direction in whichthe oxygen is supplied from the atmosphere electrode layer 34 to theexhaust electrode layer 33. In this way, in the case where the richinjection control is performed, the rich responsiveness is deteriorated.In this regard, it is recommended that the amount of the constantcurrent be a specified value determined previously.

Further, in the case where a determination result in all of the steps201 to 203 is “NO”, the routine proceeds to step 207 in which a controlnot changing the sensing responsiveness of the oxygen sensor 28 withrespect to the reference responsiveness, that is, a control forcontrolling the constant current “Ics” to “0” is performed.

In the routine shown in FIG. 13, all pieces of the processing (steps201, 204) of increasing the rich responsiveness of the oxygen sensor 28in the case where the engine 11 is in the cold state, the pieces ofprocessing (steps 202, 205) of increasing the lean responsiveness of theoxygen sensor 28 in the case where the engine 11 is in the high-loadoperating state, and the pieces of processing (steps 203, 206) ofdeteriorating the rich responsiveness of the oxygen sensor 28 in thecase where the rich injection control is performed are performed.However, the routine is not limited to this but any one or two of thesepieces of processing may be performed.

In the present embodiment 2 described above, in the system having theoxygen sensor 28 arranged downstream of the three-way catalyst 18, it isdetermined whether or not the change request for changing the sensingresponsiveness of the oxygen sensor 28 is made and the constant currentcontrol is performed on the basis of a determination result of thechange request to thereby change the sensing responsiveness of theoxygen sensor 28. Hence, the exhaust emission can be reduced byeffectively utilizing the three-way catalyst 18.

In the embodiment 2 described above, the oxygen sensor 28 is arrangeddownstream of the three-way catalyst 18. However, the oxygen sensor 28is arranged at a middle position in the three-way catalyst 18 (forexample, at a middle position between an inlet and an outlet of thecatalyst 18).

Further, in the respective embodiments 1, 2 described above, theconstant current circuit 27 is connected to the atmosphere electrodelayer 34 of the oxygen sensor 28 (sensor element 31), but theconfiguration is not limited to this. For embodiment, the constantcurrent circuit 27 is connected to the exhaust electrode layer 33 of theoxygen sensor 28 (sensor element 31) or the constant current circuit 27is connected to both of the exhaust electrode layer 33 and theatmosphere electrode layer 34 of the oxygen sensor 28 (sensor element31).

Still further, in the respective embodiments 1, 2 described above, thepresent disclosure is applied to the system using the oxygen sensor 28having the sensor element 31 of the cup type structure but a system towhich the present disclosure is applied is not limited to this. Forembodiment, the present disclosure may be applied to a system using anoxygen sensor having a sensor element of a laminated structure.

Still further, the present disclosure may be applied not only to theoxygen sensor but also to a gas sensor other than the oxygen sensor, forexample, an air-fuel sensor for outputting a linear air-fuel ratiosignal according to an air-fuel ratio, an HC sensor for sensing an HCconcentration, and a NOx sensor for sensing a NO_(x) concentration.

1. An emission gas cleaning device of an internal combustion enginehaving a catalyst which cleans an emission gas of an internal combustionengine, and an emission gas sensor which is arranged on a downstream ofthe catalyst or in the catalyst and senses a concentration of aspecified component in the emission gas by a sensor element having asolid electrolyte material provided between a pair of sensor electrodesone of which is exposed to the atmosphere, the emission gas cleaningdevice of an internal combustion engine comprising: a constant currentsupply portion making a constant current flow between the sensorelectrodes to change an output characteristic of the emission gassensor; and a current control portion determining a direction of theconstant current flowing between the sensor electrodes according to achange request for changing the output characteristic of the emissiongas sensor or according to an operating state of the internal combustionengine, the current control portion controlling the constant currentsupply portion in such a way that the constant current flows in thedirection.
 2. The emission gas cleaning device of an internal combustionengine as claimed in claim 1, wherein the catalyst is a NOxstorage-reduction catalyst for adsorbing NOx in the emission gas when anair-fuel ratio of the emission gas flowing into the catalyst is lean andfor reducing, cleaning, and discharging the NOx adsorbed by the catalystwhen the air-fuel ratio of the emission gas flowing into the catalystbecomes rich.
 3. The emission gas cleaning device of an internalcombustion engine as claimed in claim 2, wherein during a leancombustion control in which an air-fuel ratio of an air-fuel mixture tobe supplied to the internal combustion engine is controlled to a leanvalue, the current control portion controls the constant current supplyportion in such a way that the constant current flows in a direction inwhich a sensing responsiveness to a lean component of the emission gassensor is improved, and during a rich combustion control in which theair-fuel ratio of the air-fuel mixture to be supplied to the internalcombustion engine is controlled to a rich value, the current controlportion controls the constant current supply portion in such a way thatthe constant current flows in a direction in which a sensingresponsiveness to a rich component of the emission gas sensor isimproved.
 4. The emission gas cleaning device of an internal combustionengine as claimed in claim 1, wherein the catalyst is a three-waycatalyst for cleaning CO, HC, and NOx in the emission gas.